Interpretive Summary: Listeria monocytogenes causes severe infections in humans such as septicemia, encephalitis, and meningitis. Individuals who are typically at risk are immunocompromised, elderly, infants, and pregnant women. Outbreaks of listeriosis have been associated with contaminated refrigerated food products, where L. monocytogenes can grow at low temperatures. Because mortality following infection with Listeria can be high, treatment with antimicrobials is warranted. Therefore, emergence of antimicrobial resistant L. monocytogenes strains compromises treatment options. The objective of this study was to determine antimicrobial susceptibility in Listeria isolated from various sources. From December, 2006 to April, 2008, L. monocytogenes, L. innocua, and L. welshimeri were isolated from humans, animals (swine feces and dairy manure), and food using standard protocols. All isolates were subjected to susceptibility testing using antimicrobials which encompass those used in treatment. Resistant isolates were analyzed for the presence of antimicrobial resistance genes. L. monocytogenes isolates were resistant to ciprofloxacin, ceftriaxone, oxacillin, tetracycline, and clindamycin. In addition to these drugs, L. innocua isolates were also resistant to penicillin, while L. welshemeri isolates exhibited additional resistant to quinupristin/dalfopristin, rifampin, penicillin, trimethoprim/sulfamethoxazole, and streptomycin. Tetracycline resistance in L. monocytogenes and L. innocua was attributed to the presence of a transferable antimicrobial resistance gene. The current study demonstrates the variability in levels of resistance between Listeria species. Furthermore, the presence of transferable resistance genes suggest that transfer between bacterial populations is possible. Additional studies are warranted to determine the frequency of resistance gene transfer between bacteria. Results from the current study will be useful for medical and veterinary personnel, animal husbandry management, and researchers studying the impact of antimicrobials on resistance in L. monocytogenes.

Technical Abstract:
The current study compared antimicrobial susceptibility of Listeria innocua, L. welshimeri and L. monocytogenes isolated from various sources. Antimicrobial susceptibility testing was performed using a microbroth procedure with Sensititre® minimum inhibitory concentration (MIC) plates containing 18 antimicrobials. Resistant isolates were analyzed for the presence of antimicrobial resistance genes using PCR. The majority of L. monocytogenes isolates were resistant to oxacillin (99%, 89/90) and ceftriaxone (72%, 65/90) while few isolates were resistant to clindamycin (21%, 19/90) and ciprofloxacin (2%, 2/90). Comparing selected sources of L. monocytogenes, resistance to ceftriaxone, clindamycin and oxacillin ranged from 27 – 86%, 7 – 43% and 96 – 100%, respectively. Resistance to ciprofloxacin (6%, 2/34), quinupristin/dalfopristin (7%, 1/14) and tetracycline (7%, 1/15) was observed with L. monocytogenes isolated from food, animal and environmental sources, respectively. All L. welshimeri isolates (6/6) were resistant to streptomycin, quinupristin/dalfopristin, ciprofloxacin, rifampin, oxacillin, penicillin and clindamycin, while most isolates (67%, 4/6) were resistant to trimethoprim/sulfamethoxazole. All L. innocua isolates (4/4) were resistant to oxacillin and pencillin, whereas 75% (3/4) of isolates were resistant to tetracycline, ceftriaxone and clindamycin. Resistant isolates were negative for aadA, strA – B, sul I – II, penA, vat(A – E), vga(A – B) and vgb(A – B). However, tetM was detected among tetracycline resistant isolates. L. welshimeri was resistant to more of the tested antimicrobials as compared to the other two Listeria species tested, but resistance was not attributed to selected resistance genes. These data demonstrate the variability in resistance among Listeria species. However, the human pathogen, L. monocytogenes, appears to be the least resistant among the tested species.